1. Field of the Invention
The present invention relates to a method for producing a hot rolled steel sheet or strip having high tensile strength, high ductility and low yield ratio.
Conventionally, high tensile strength steels have mostly been used as structural thick gauge steel plate, but in recent years in various industries, such as the automobiles, railways and industrial machines. The increasing tendency is that the hot rolled high tensile strength steel sheet is used more and more for the purpose of weight-savings and cost reduction. However, in the fields of industry where a relatively thin gauge high tensile strength steel sheet is used, the steel sheet is very often press-formed. Therefore, conventional high tensile strength steels, having a high yield strength, hence a high yield ratio, have been confronted with various problems such that they can not be severely worked due to their low ductility, that accuracy in the formed articles produced from these materials is often unsatisfactory due to their spring-back phenomena after their deformation, and that wearing of tools is substantial and dies are easily worn due to their high yield strength.
Thus increasing demands have been made among various users for development of a low yield ratio but high tensile strength steel sheet which shows a high degree of work hardening and a satisfactorily high yield point, or yield strength after press-forming.
2. Description of Prior Art
A typical low yield ratio, high tensile strength steel sheet which has been demanded in the above fields of industries must show a tensile strength not lower than 50 kg/mm.sup.2 and a yield ratio not higher than 70% as well as excellent ductility in the case of a thin gauge steel sheet, for example, of thickness of 4 mm or less. Steel materials suitable for producing such a steel sheet as above include a bainite steel. This bainite steel, as well known, is a high strength steel utilizing the high strength of the bainite which is a decomposition product of austenite at low temperatures.
For rolling this bainite steel by means of an ordinary hot strip mill, it is necessary to increase the manganese content and add elements, such as Ni, Cr and Mo, in addition to the precipitation hardening elements, such as Nb, V and Ti, in order to improve the hardening. Therefore, the production cost is inevitably increased due to the additional elements so that this steel material has been confronted with disadvantages when it is used as automobile steel sheets which must be produced at a low production cost.
Also, the carbide and nitride forming elements, such as Nb, V and Ti, which are added in the conventional high tensile strength steels for the purpose of improving the strength and toughness are undesirable because they increase the yield ratio, usually 80% or higher.
Meanwhile trials have been proposed to lower the yield ratio by annealing the hot rolled steel coil at a temperature higher than the transformation temperature in a heat treating equipment, such as a continuous annealing line, and rapidly cooling the coil. However, this process requires a separate process of heat treatment, thus increasing the production cost.